One of the challenges during drilling is to identify zones of interest in the formation. Zones of interest are usually distinguished by their permeability relative to the entire formation through which the drilling is occurring. Zones of high permeability contain highly porous rock, having interconnections between the pores, which allow the hydrocarbons residing within the rock to flow through and out of the rock. Suitable reservoir rock is therefore porous, permeable and contains sufficient hydrocarbon to make it economically feasible to drill and produce them.
Ideally, the identification of zones of interest should occur at the time of drilling, so as to be most efficient and cost effective, however most methods of determining permeability are rarely determined while drilling.
One prior art method is to perform a drill stem step in which the drill bit is tripped out, a tool is run in and pressure over time is monitored. Dril stem tests, while usually effective and time consuming and expensive. Others apply qualitative geological analysis of the drilling mud cuttings for a tough yet often very inaccurate assessment of the formation characteristics.
It is known to pump drilling mud downhole through the drill string to the bit to flush cuttings and hydrocarbons from around the bit. The mud is flowed up the annulus between the wellbore and the drilling string, to the surface for removal of solids and cuttings in an active mud system. The recovered mud, containing the cuttings, is flowed across a shale shaker where large solids are removed and can be sampled. A mud tank containing cleaning systems, such as sumps or centrifuges, are used to remove the fine particulates. The cleaned mud can then be recirculated downhole.
The solids in the mud returning from the wellbore are representative of the formation and can be analyzed for a number of characteristics indicative of it's hydrocarbon producing ability.
Some quantitative values for drilling have already available through observation of drilling performance, including such parameters as force of the bit (FOB) and bit revolutions per minute (RPM). Drilling through highly porous and possibly permeable rock results in an increased RPM and a decrease in the FOB. Additionally, the rate of penetration (ROP) is determined and is much faster through porous rock. Other methods and equipment exist for the measurement of hydrocarbon content in porous and permeable rock and require sophisticated on-site equipment and monitoring devices.
In its simplest form, conventional analysis has been performed qualitatively, largely by visually assessing characteristics known to occur in suitable reservoir rock. Such characteristics include the size of particles present, the angularity of the particles, the degree to which the particle sizes are similar (sorting), the degree to which the particles bind together (cementation), and the porosity of the rock and have been individually assessed by many different means. Industry standards have been set for these qualitative analyses, however they remain, to a large degree, subjective.
As sampling is normally done at defined drilling depths, characteristics can be indexed to the drilling depth using the lag time required to bring the mud to the surface. The zones of interest can then be identified on well logs coordinated with the depth and lithology of the sample.
Because of the subjective nature of visual analysis, even though attempts have been made to set standards, it would be advantageous to provide an easily calculated quantitative or semi-quantitative index that utilizes all of the simply assessed characteristics of the cuttings to determine a relative index of permeability.